1. Field of the Invention
The present invention relates to an exhaust system having a catalytic converter built therein for removing obnoxious emission components from exhaust gases from a combustion engine and also for reducing engine exhaust noises. The present invention relates also to a motorcycle having the exhaust system referred to above.
2. Description of the Prior Art
The motorcycle exhaust system with a catalytic converter built therein and fluidly connected with the motorcycle combustion engine is generally so designed and so structured that exhaust gases introduced towards a muffler through an exhaust tube that is fluidly connected with an exhaust port of the motorcycle engine may be first introduced into an expansion chamber, defined in an upstream region of an exhaust passage within the muffler, and be subsequently passed through a catalytic converter disposed downstream of the expansion chamber with respect to the direction of flow of the exhaust gases towards the atmosphere. In this motorcycle exhaust system, it is well known that as the exhaust gases flow through the expansion chamber within the muffler, the exhaust gases can be expanded, accompanied by reduction of the exhaust energies and flow velocity of the exhaust gases so that the flow resistance of the exhaust gases ready to pass through the catalytic converter can be reduced.
In the catalytic converter-incorporated exhaust system of the kind discussed above, if the total quantity of oxygen contained in the exhaust gases can be utilized to reburn unburned obnoxious components of the exhaust gases that are left as the exhaust gases pass through the catalytic converter, the exhaust gases can be substantially purified to such an extent as to contain no substantial quantity of the obnoxious components, with the catalytic converter thus exhibiting an excellent purifying performance. In order to accomplish the substantial purification of the exhaust gases in this way, it is desirable to control the engine exhaust gases to contain a predetermined or required quantity of oxygen and, specifically, to control the air-fuel mixture to have an air/fuel mixing ratio that is about equal to the stoichiometric air/fuel mixing ratio of 14.8. In view of this, the Japanese Laid-open Utility Model Publication No. 6-18639, first published Mar. 11, 1994, for example discloses the use of a sensor for detecting the amount of oxygen contained in the exhaust gases introduced into the muffler so that a fuel supply control device can control, on a feedback scheme, the amount of fuel to be supplied to the combustion engine in dependence on a detection signal generated by the sensor.
However, with the prior art catalytic converter-incorporated exhaust system discussed above, the exhaust gases are introduced into and then purified substantially within the catalytic converter after it has first been introduced into the expansion chamber. Since the exhaust gases expanded within the expansion chamber have their flow velocity lowered on one hand and have a static pressure increased on the other hand and, therefore, a portion of the exhaust gases of the increased pressure tends to leak outwardly through a gap between a partition wall, used in the muffler to form the expansion chamber, and an inner surface of the muffler casing to such an extent that the total amount of the exhaust gases cannot be passed through the catalytic converter, resulting in reduction of the exhaust gases purifying performance. On the other hand, if the sensor for detecting the amount of oxygen contained in the exhaust gases is disposed within such expansion chamber positioned at a location immediately upstream the catalytic converter to detect the amount of oxygen contained in the exhaust gases, the amount of oxygen contained in the exhaust gases at the time they have been just exhausted from the vehicle engine cannot be detected accurately because oxygen contained in the exhaust gases then entering the expansion chamber is mixed within the expansion chamber with oxygen which was contained in the preceding exhaust gases and was left within such expansion chamber. As a result, the feedback control of the amount of fuel to be supplied in dependence on the detection signal outputted from the sensor will have a low or slow response and the amount of oxygen contained in the exhaust gases then passing through the catalytic converter may not attain a required value, with the consequence that the exhaust gases may not be substantially purified sufficiently.